On a separate novice carb question, what is effected when you change needle position? I've always matched settings or done it by the book.

disston

04-30-2013 08:02 AM

Increasing the jet sizes is what is noticed when the carb is used on a larger engine usually. Larger jets make for a richer gas mixture and these increases are incremental. The larger engine will need the extra gas but the carb set up for a large engine will be too rich on a smaller engine.

The jet needle is inside the needle jet and the needle jet can be increased. Those jets usually come in sizes thousands apart. I'm familiar with sizes 2.66 and 2.68. So an increase of the needle jet from 2.66 to 2.68 would be one jump in size. It is said that raising the needle one notch is a jump in 3 jet sizes.

These are not really hard fast rules about carbs and sizes. But the idea is that raising the needle is a bigger change than a jump in jet sizes.

You want to look at the spark plugs and see how rich/lean the carbs are running. This is tricky because the engine runs on different parts of the carb at different rpms. We do have idle jets and main jets and the needle jet/jet needle. They each work and have more influence at different rpms but they also work together.

I'm trying to not get myself in trouble here by writing a bunch of stuff that isn't really accepted. I don't mess with the jets in my carbs very often but I have tried a couple of things. I don't have the jet figures you want, in other words. I have heard of riders that got satisfaction from raising the jet needle. But I think I would have to lower something else on my bike if I did that.

Finally I will say that this was easier before we had to deal with Gasohol. It doesn't burn the same and it also doesn't leave the same color on the plugs, IMHO.

I would also suggest finding some legitimate Hot Rod or Racing forums and read what these guys do. And I bet you could find a bunch on just Carburetor Theory from a bunch of frustrated Physicists.

supershaft

04-30-2013 11:53 AM

The later model bikes from R65 to R100 are jetted with the same mains and just about every other jet in their carbs and they are all jetted very well. Different engines will pull different amounts of fuel through the same size jets.

Changing jet needle positions is not like changing the needle jet size two or three sizes. The are different jets and effect different parameters.

B_C_Ries

04-30-2013 01:54 PM

Here is my favorite way to jet Carburetors, firsts off you need to have the bike properly timed and good sparkplugs.

either go to a drag strip that has bracket racing or get a friend or two and a stop watch.

Find out your 1/4 mile time with existing main jets and with jets that are richer and leaner, (be certain to always shift at the exact same RPM). This actually works best if there is a mild uphill slope. If leaner jets make you faster then ge even leaner until you start to go slower, same deal if richer, richen it up until it slows down, then go back to where it was fastest.

next to set your needle, put some masking tape on your throttle grip and another on the switch housing by your throttle grip, draw a line which matches up when you are close to 1/2 throttle, time yourself but only use 1/2 throttle as indicated by your marks on the throttle, raise the needle and check your time again, try the different needle positions and use the one that gives you the best time.

If you can't get to a dragstrip and all your friends are busy you can test your accelleration between two points, ie:find two signs or other obvious objects about 1/8th of a mile or less apart, approach at 40 mph in 4th or 5th gear then open it up all the way as you pass the first mark point then observe your speed as you pass the second point, to set your needle position use 1/2 throttle, make sure you are using the same ammount of throttle for each test run.

again if you can, going uphill will be a good idea as it will reduce your top speed.

After you get your mains and needle position sorted out, pick a low speed jet that allows you bike to idle best with the idle mixture screw set between 3/4 to 2 turns out.

carry paper and write down your results, including date, temperature, weather and time of day. The jets effect each other so expect that the best idle jet may change as you use different mains and move the needle,

disston

04-30-2013 02:56 PM

I said I thought there was a correlation between engine size and jet sizes. I was looking at the info wrong I think. There seems to be a correlation between age and jet size. The older bikes are jetted richer. But then on reflection I don't think it's the age but the compression ratio. The higher compression bikes seem to have bigger jets.

SS does much more rejetting of Airhead carbs than I do so I appreciate his input.

The later bikes also had leaner jets because of emission concerns.

This whole subject is very confusing because of the numerous variations BMW used. I just spent an hour trying to find the part numbers for every main jet BMW ever used on the Airheads. There are 14 main jet sizes. There are 5 needle jet sizes. At least 2 idle jet sizes. (These numbers are for machines sold in N America. There are some other sizes for European machines)

Anybody want me to type up the list for reference?

B_C_Ries

04-30-2013 04:35 PM

there is an air jet or passage that interacts with the fuel jet, different carburetors may have different sized air passages,

I've jetted Weber Carburetors and they have interchangable air jets, most carburetors just have an air passage of a fixed size, brave tuners can drill out these passages to effect mixture,

A carburetor can have smaller air passages or jets and small fuel jets and run richer than a carburetor with larger fuel jets and big air jets,

Herr Bing was kind enough to put numbers on each carburetor so you have a clue if something might be different inside

supershaft

04-30-2013 04:56 PM

Quote:

Originally Posted by disston
(Post 21303210)

I said I thought there was a correlation between engine size and jet sizes. I was looking at the info wrong I think. There seems to be a correlation between age and jet size. The older bikes are jetted richer. But then on reflection I don't think it's the age but the compression ratio. The higher compression bikes seem to have bigger jets.

SS does much more rejetting of Airhead carbs than I do so I appreciate his input.

The later bikes also had leaner jets because of emission concerns.

This whole subject is very confusing because of the numerous variations BMW used. I just spent an hour trying to find the part numbers for every main jet BMW ever used on the Airheads. There are 14 main jet sizes. There are 5 needle jet sizes. At least 2 idle jet sizes. (These numbers are for machines sold in N America. There are some other sizes for European machines)

Anybody want me to type up the list for reference?

Higher compression does call for richer jetting sometimes but the main difference between the older bikes with 9.0:1 CR and the newer ones with 8.5:1 CR is that the earlier bikes were jetted too rich. Jetting mains? My bikes and most all setups I have worked on will go faster and faster doing tests somewhat like BCR's while you are jetting them leaner and leaner. Up until the point of not running at all at higher rpm. Personally, I don't think BCR's tests work that well for the other jets. I don't think the manual's theoretical throttle position/jet parameter works out in reality. What does work? Trying different jetting combo's until you find what works best as lean as it will work best.

B_C_Ries

04-30-2013 05:44 PM

It worked out really well on a 2 stroke triple with mikunis, also worked great on a GS 750E with an 840 kit with 11 to 1's and Andrews cams that put max power at 14,000 rpm, power wheelies at 100 mph. I also have alot of experience setting up Holleys on muscle cars, but unless you get brave with a drill you mainly mess with main jets, power valves, accelerator pumps and secondary springs, no specific midrange jetting adjustment.
But there might be some different issues with CV carbs.

I haven't jetted My R100/7 because it seems to be running real well with the jets it came with,

I suppose it is worth mentioning that I was really into hotrodding/ motorcycles and street racing in the late 70's real early 80's. I did most of my performance tuning and racing at night. If you jet for cold night air you will go richer as it warms up. And richer mixtures will keep your valves/ heads and piston tops cooler. On the other hand if you get things perfect when it is hot outside you can go lean and burn up a motor as it cools off outside and air becomes more dense.

Beemerboff

05-02-2013 07:05 AM

Problem with tuning CV carbs with white markers on the twistgrip is that you have no way of knowing where the piston/slide/needle is, as the throttle is only connected to the butterfly.

The position of the slide is dependent on the vacuum above it, and the strength of the spring above it which slows down the vacuum induced movement.

The mixture in transition is not solely dependent on the needle/ jet relationship either - if the butterfly opens faster than the slide opens the flow is faster above the jet outlet, which draws up more fuel and richens the mixture. So the strength of the spring affects the mixture too.

The later piston springs have a different part number, and are noticeably stiffer than the early ones, so if you use the later jetting at least try the spring that is listed with the jetting - getting the spring right can make a big difference.